Non-ambipolar radio-frequency plasma electron source and systems and methods for generating electron beams

ABSTRACT

An electron generating device extracts electrons, through an electron sheath, from plasma produced using RF fields. The electron sheath is located near a grounded ring at one end of a negatively biased conducting surface, which is normally a cylinder. Extracted electrons pass through the grounded ring in the presence of a steady state axial magnetic field. Sufficiently large magnetic fields and/or RF power into the plasma allow for helicon plasma generation. The ion loss area is sufficiently large compared to the electron loss area to allow for total non-ambipolar extraction of all electrons leaving the plasma. Voids in the negatively-biased conducting surface allow the time-varying magnetic fields provided by the antenna to inductively couple to the plasma within the conducting surface. The conducting surface acts as a Faraday shield, which reduces any time-varying electric fields from entering the conductive surface, i.e. blocks capacitive coupling between the antenna and the plasma.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/427,273, now U.S. Pat. No. 7,498,592 filed Jun. 28, 2006, thedisclosure of which is incorporated herein by reference in its entirety.

This invention was made with United States government support awarded bythe following agencies:

DOE DE-FG02-97ER5

NASA NNC04GA82G and T302-9700

The United States government has certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is directed to systems, methods and devices forgenerating an electron beam.

2. Related Art

Electron beam sources are widely used in a variety of applications.Electron beam generators are used both as sources for the electron beamsthemselves, as charge neutralizers for charged ion beams, to produceprotective thermal spacecraft coatings, to form plasma-assisted thinfilms, and to deposit optical coatings, such as, for example, for largemirrors, in forming metallized packaging films and in electron beamevaporation, electron beam surface modification, thin film growth,plasma-assisted chemical vapor deposition, plasma vapor deposition,electron beam curing, waste handling, and electron beam reactivedeposition.

Ion beams are used both in the semiconductor manufacturing industry andmany other industries, as well as in many satellites and otherspacecraft, and other applications. In such satellites and otherspacecraft, ion beams are used as thrusters to maneuver the satellitesor other spacecraft. In the semiconductor industry, ion beams are usedfor a variety of purposes, including etching, ion implantation, doping,sputtering, and the like.

In both semiconductor manufacturing and spacecraft/satellite maneuveringembodiments, it is highly desirable, if not absolutely necessary, thatthe plasma stream, i.e., the ion beam, be electrically neutral. The ionbeams are typically generated by stripping electrons off of atoms of thedesired material to create positively-charged ions. Thesepositively-charged ions are accelerated by an electric field and formedinto a beam. Typically, the positively-charged ions originate in aplasma.

However, due to space-charge limitations within the ion beams, thecharged ions in the ion beams tend not to stay tightly packed in thebeam. Rather, the ion beam tends to “blow apart” due to the repulsiveforce between the similarly-charged ions. Furthermore,positively-charged ion beams are attracted to negatively-chargedsurfaces. For example, in the spacecraft/satellite embodiments, if thebeam remains positively-charged, two problems arise. First, thespacecraft/satellite itself becomes negatively charged when the positivecharge is emitted. Second, because the ion beam is positively charged,it becomes attracted to the negatively-charged spacecraft/satellite, andthus does not travel in a straight line away from the spacecraft orsatellite, or, in a worst-case leave the spacecraft environment at all.Rather, the positively-charged ions move within the electric fieldformed by the negatively-charged spacecraft/satellite and return towardthe spacecraft/satellite due to the electrostatic attraction between thenegatively-charged spacecraft/satellite and the positively-charged ions.As a result, a positively-charged ion beam does not provide the properthrust to appropriately maneuver the satellite or spacecraft.

Typically, to avoid these problems, the positively-charged ion beam isneutralized shortly after it leaves the ion beam generating device bycombining the positively-charged ion beam with a beam of(negatively-charged) electrons. The combination of the electrons andpositively-charged ions renders the net plasma stream neutrally charged.However, because of the relatively light weight of the electrons,relative to the ions, the electrons do not significantly affect thethrust provided by the ion beam. Moreover, by extracting equal currentsof ions and electrons, no net charge accumulates in and/or on thespacecraft/satellite. Because the ions in the plasma stream are nowbalanced by electrons, a net electric field does not arise on thespacecraft or satellite. Thus, the plasma stream moves in a straightline away from the satellite or spacecraft, providing the desiredthrust.

SUMMARY OF THE DISCLOSED EMBODIMENTS

Conventionally, electron beams associated with spacecraft are generatedby hollow cathodes. However, hollow cathodes are problematic for anumber of reasons. First, as the hollow cathodes are used to generatethe desired electron beam, they are slowly consumed. Typical maximumlifetimes for commercial hollow electrodes are on the order of onlythree to four years. Additionally, the present generation of hollowcathodes employ barium oxide-tungsten (BaO—W) inserts as their emittingsurface. However, this emitting surface deteriorates over time. Once thehollow cathode becomes inoperable, it is no longer possible to use theelectron generating device. Additionally, hollow cathodes are difficultto ignite, either initially or if they should go out during use, and canbecome contaminated, thus reducing their efficiency.

One proposed solution for this limited lifetime is to provide multiplehollow-cathode electron generating devices and/or to provide multiplehollow cathodes within a single hollow-cathode electron generatingdevice. However, these solutions are problematic for a number ofreasons. First, for weight-limited devices such as satellites andspacecraft, providing two electron generating devices consumes valuableand limited weight and space within the spacecraft/satellite. Second,even when two such hollow-cathode electron generating devices areprovided, it has not always been easy to ignite the hollow cathode inthe second hollow-cathode electron generating device. This is also truewhen multiple hollow cathodes are provided in the same hollow-cathodeelectron generating device.

While hollow cathode-electron generating devices have limited usefullifespans and the other problems outlined above, they are generallywell-understood devices that reliably provide electron beams over theirlifetimes. Any competing technology should be at least as useful,reliable, and efficient or long-lived as hollow cathode devices to becommercially successful.

This invention provides an electrode-less electron beam generatingdevice.

This invention separately provides systems and methods for providingnon-ambipolar electron flow in an electron generating device.

This invention separately provides systems and methods for providingtotal non-ambipolar electron flow in an electron generating device.

This invention separately provides systems and methods for creating anelectron-generating plasma using magnetic induction to generate currentsin the plasma.

This invention separately provides systems and methods for creating anelectron-generating plasma using helicon-wave induction fields togenerate currents in the plasma.

This invention separately provides systems and methods for improvingelectron extraction in an electron beam generating device.

This invention separately provides systems and methods for gridlessnon-ambipolar electron extraction of electrons from an electron beamgenerating device.

This invention separately provides systems and methods for extractingelectrons from an electron beam generating device through an electronsheath.

In various exemplary embodiments of systems, methods and/or devicesaccording to this invention, an electron beam generating device produceselectron beams from a plasma, where the plasma is produced usingradio-frequency (RF) fields and electron extraction occurs throughelectron sheaths. In various exemplary embodiments, an ion loss area isselected based on an electron extraction area, the ion mass and theelectron mass. In various exemplary embodiments, the ion loss area issufficiently large to allow for total non-ambipolar electron extraction.In various exemplary embodiments, the ions are lost to anegatively-biased conducting surface. In various exemplary embodiments,the negatively-biased conducting surface is a cylinder. In variousexemplary embodiments, electrons are extracted through a grounded ringthat is mounted in or behind an insulating boundary provided at one endof the conducting cylinder. In various exemplary embodiments, theelectrons extracted from the plasma pass to or through the groundedring, while the ions are lost to the negatively-biased conductingsurface. In various exemplary embodiments, an axial magnetic field thatis parallel to the axis of the ring is used to enhance electronextraction through the ring. In various exemplary embodiments, the axialmagnetic field also reduces the electron current to the ring itself.

In various exemplary embodiments, an antenna located outside of thenegatively-biased conducting surface generates a varying RFelectromagnetic field around the electron beam generating device. Theantenna can be capacitively coupled to the plasma, inductively drivingcurrents in the plasma or inductively exciting helicon waves, providedin the negatively-biased conducting cylinder depending on the structureof the device and the plasma density. In various exemplary embodiments,slots or other voids in a negatively-biased conducting cylinder to allowthe time-varying magnetic fields provided by the antenna to extend intothe interior of the negatively-biased conducting cylinder to inductivelycouple to the gas within the negatively-biased conducting cylinder. Invarious exemplary embodiments, the negatively-biased conducting cylinderacts as Faraday shield to reduce, and possibly eliminate, any capacitivecoupling of electric fields between the antenna and the plasma. Invarious exemplary embodiments, a simple antenna is used In various otherexemplary embodiments, the antenna is configured to allow inductive orhelicon coupling to the plasma.

In various exemplary embodiments, a non-conducting closed surface isplaced around the negatively-biased conducting cylinder to confine theplasma and a source gas. In various exemplary embodiments, electronextraction aperture dimensions of the grounded electron extraction ringand the gas flow rate into the chamber determine the appropriate neutralgas pressure within the electron beam generating device. In variousexemplary embodiments, any neutral gas can be used.

In various exemplary embodiments, the device can be operated with avariety of non-time-varying (DC) magnetic field configurations. Givensufficient RF power, such steady-state or DC magnetic fields allowhelicon waves to be excited within the plasma in the interior of theelectron beam generating device. Helicon waves allow the extractedelectron current to be increased due to increases in the plasma density.In various exemplary embodiments, the steady-state or DC magnetic fieldsare aligned axially. In various exemplary embodiments, the steady-stateor DC axial magnetic fields are produced by permanent magnets and/or byelectromagnets.

These and other features and advantages of various exemplary embodimentsof systems and methods according to this invention are described in, orare apparent from, the following detailed descriptions of variousexemplary embodiments of various devices, structures and/or methodsaccording to this invention.

BRIEF DESCRIPTION OF DRAWINGS

Various exemplary embodiments of the systems and methods according tothis invention will be described in detail, with reference to thefollowing figures, wherein:

FIG. 1 is a side-cross sectional view of a first exemplary embodiment ofan electron beam generating device according to this invention;

FIG. 2 is a plan end view of a first end of the first exemplaryembodiment of the electron beam generating device shown in FIG. 1;

FIG. 3 is a plan view of the other end of the first exemplary embodimentof the electron beam generating device shown in FIG. 1;

FIG. 4 is side perspective view of the first exemplary embodiment of theelectron beam generating device shown in FIG. 1;

FIG. 5 is a side-cross sectional view showing in greater detail a firstexemplary embodiment of the extraction end of the first exemplaryembodiment of the electron beam generating device shown in FIG. 1;

FIG. 6 is a side cross-sectional view showing in greater detail oneexemplary embodiment of the supply end of the first exemplary embodimentof the electron beam generating device shown in FIG. 1;

FIG. 7 is a side cross-sectional view showing in greater detail a secondexemplary embodiment of the extraction end of the first exemplaryembodiment of the electron-beam generating device shown in FIG. 1;

FIG. 8 is side-cross sectional view of a second exemplary embodiment ofan electron beam generating device according to this invention;

FIG. 9 is an end plan view of one end of the second exemplary embodimentof the electron beam generating device shown in FIG. 8;

FIG. 10 is a side cross-sectional view of a third exemplary embodimentof an electron-beam generating device according to this invention;

FIG. 11 is a schematic view of one exemplary embodiment of an electronbeam generating device and antenna drive circuit according to thisinvention;

FIGS. 12-15 show a plurality of different antenna designs useable tocreate a plasma within the first and second exemplary electron beamgenerating devices shown in FIGS. 1, 8 and 10;

FIG. 16 is a flowchart outlining one exemplary embodiment of a methodfor generating and extracting an electron beam according to thisinvention.

FIG. 17 is a graph of the electron/particle current ratio as a functionof radio-frequency power and gas flow rate;

FIG. 18 is a graph of the generated currents as a function of magneticfield strength; and

FIG. 19 is a graph of the plasma potential as a function of radialdistance from the axis of the ion collection surface.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Ion and Hall thrusters use beams of positively-charged ions forpropulsion. As discussed above, electrons or negative ions should beintroduced into the positively-charged ion beam as it leaves thethruster. This is done to prevent the spacecraft from becomingnegatively charged and thus attracting the emitted positively-chargedion beam.

Traditionally, hollow cathodes have been used as neutralizing sourcesbecause of their high electron current density and relatively low powerrequirements. However, the operational lifetime of such hollow cathodesis limited by cathode deterioration, cathode contamination, and othereffects. This limited operational lifetime for hollow cathodes rendershollow cathodes less suitable for sustained use or where maintainingsuch hollow cathodes is difficult or impossible.

Longer duration spacecraft missions that use ion propulsion, such as theproposed Jupiter Icy Moons Mission (JIMO), will take 6-10 years for thetotal orbital transfer time. While using ion propulsion for such longerduration missions is very beneficial relative to impulsive chemicalrocket burns, due to the savings in fuel mass and time, the lifetime forsome operating components for ion propulsion, such as the hollowcathodes, may be limited to no more than 3 to 4 years. The hollowcathode neutralizer and plasma sources that were used for the highlysuccessful Deep space 1 and SMART-1 missions were limited to no morethan 3 to 4 years of operational lifetime due to significant erosion,sputtering and re-deposition of material within the keeper region andsurrounding area of such devices.

The inventors have determined that radio-frequency (RF) plasmas areattractive as sources for neutralizing charge carriers for electricpropulsion devices, such as Hall and ion thrusters. Such radio-frequencyplasmas allow for an electrode-less design and provide high efficiencyand long operational lifetimes. Radio-frequency plasma sources providean alternative neutralizing approach that does not consume electrodematerial, while providing electrons, allowing for a longer operationallifetime.

There are a variety of radio-frequency plasma sources, includingcapacitive and inductive sources, that operate without magnetic fields,and electron cyclotron resonance (ECR) sources and helicon sources thatrequire axial magnetic fields. Helicon sources can produce the highestplasma densities, which can be greater than 10¹³/cm³, for a givenradio-frequency power. However, helicon sources also require magneticfields. Lower RF power emitted by the excitation antenna into the plasmarequires higher magnetic field strengths. For example, a 10 Wradio-frequency signal typically requires a 2000 Gauss magnetic field.In contrast, lower magnetic field strengths require higher RF power intothe plasma. For example, a 300 Gauss magnetic field typically requiresthe excitation antenna to emit 600 W. If sufficient power is notavailable, helicon sources will operate as inductive sources.Inductively coupled plasmas can achieve significant plasma densities,such as, for example, 10¹⁰/cm³ to 10¹²/cm³ and allow for a large totalelectron extraction current.

FIG. 1 shows a first exemplary embodiment of an electron beam generatingdevice according to this invention that is useable to generate aradio-frequency plasma that provides a beam of electrons without anelectrode. As shown in FIG. 1, in various exemplary embodiments, theelectron beam generating device 100 comprises a generally non-conductiveexterior chamber 110, one or more steady-state magnets 120, a conductiveion-collection surface 130, a radio-frequency antenna 140, an electronextraction ring 150, and connections 162 and 166 to a negative voltagesource 160 and a reference ground voltage 164, respectively. FIGS. 2 and3 show exterior plan views of supply and extraction end walls 114 and118, respectively, of the non-conductive exterior chamber of theelectron beam generating device 100. FIG. 4 shows a side perspectiveview of the electron beam generating device 100.

As shown in FIG. 1, the non-conductive exterior chamber 110 comprises anon-conductive chamber surface 112, a non-conductive supply end wall114, and a non-conductive extraction end wall 118. A gas supply tube 116extends through the supply end wall 114 and into an interior of theconductive ion-collection chamber 130. The gas supply tube 116 suppliesa feed gas 102 at least into an interior space 106 that is enclosed bythe non-conductive exterior chamber 110. During operation, the electronbeam generating device 100 forms a plasma 108 within in the interiorspace 106 such that an electron beam 104 passes, along with the feed gas102, through a central aperture, hole or passageway 152 in the electronextraction ring 150. The electron extraction ring 150 is located in, andextends through, the extraction end wall 118 into the interior space106.

As shown in FIG. 1, in various exemplary embodiments, the one or moresteady-state magnets 120, such as the cylindrical steady-state magnet122, are arranged such that the south (or north) poles of the one ormore steady-state magnets 120 face the supply end wall 114, while thenorth (or south) poles of the one or more steady-state magnets 120 facethe extraction end wall 118. It should be appreciated that it is notimportant that the north end faces the extraction side. However, the oneor more steady-state magnets 120 should produce a magnetic field that isaligned with the axis of the center extraction aperture or passageway152. As shown in FIG. 1, in various exemplary embodiments, the at leastone steady-state magnet 120 can be located outside of and surroundingthe non-conductive exterior chamber 110.

It should be appreciated that, in various exemplary embodiments, thenon-conductive exterior chamber 110 is cylindrical in cross section.Accordingly, in such exemplary embodiments, the at least onesteady-state magnet 120 has a corresponding cylindrical central openingthrough which the non-conductive exterior chamber 110 extends. However,it should be appreciated that, in various other exemplary embodiments,the non-conductive exterior chamber 110 can have any desiredcross-sectional shape that defines a simple closed curve, such as acircle, a regular or irregular polygon or the like. Typically, the oneor more steady-state magnets 120 will be placed around thenon-conductive exterior chamber 110 such that the central passagewayformed within the at least one steady-state magnet 120 will closelyfollow the surface of the non-conductive exterior chamber 110.

The one or more steady-state magnets 120 generate a generally solenoidalmagnetic field that extends along the axial direction of thenon-conductive exterior chamber 110. In various exemplary embodiments,such as that shown in FIG. 1, the at least one steady-state magnet 120comprises only a first steady-state magnet 122. In various exemplaryembodiments, the first steady-state magnet 122 is a permanent magnet.However, in various other exemplary embodiments, the one or moresteady-state magnets 120 can be electromagnets provided with asteady-state or DC electric current.

As shown in FIG. 1, the conductive ion-collection surface 130 isprovided adjacent to at least the extraction end wall 118 within theinterior space 106 provided in the non-conductive exterior chamber 110.Likewise, as shown in FIG. 1, in various exemplary embodiments, theconductive ion-collection surface 130 closely follows the interiorsurface of the non-conductive exterior chamber 110. The radio-frequencyantenna 140 is placed around the exterior of the non-conductive exteriorchamber 110. The location for the radio-frequency antenna 140 allows theradio-frequency electric and/or magnetic fields generated by placing aradio-frequency signal onto the radio-frequency antenna 140 to interactwith the feed gas 102 to create the plasma 108. In various exemplaryembodiments, the radio-frequency antenna 140 is formed from a singleturn of water-cooled copper pipe and can operate at radio frequenciesnormally less than the electron cyclotron frequency, where the electroncyclotron frequency f_(c) is:

${f_{c} = \frac{eB}{2\;\pi\; m_{e}}},$where:

e is the electron charge;

B is the magnetic field strength; and

m_(e) is the electron mass.

As shown in FIG. 1, in various exemplary embodiments, the conductiveion-collection surface 130 has a plurality of slots or voids 132 formedin it. In various exemplary embodiments, these slots or voids 132 extendfrom the end of the conductive ion-collection surface 130 that isadjacent to the extraction end wall 118 inwardly toward the other end ofthe conductive ion-collection surface 130. In various exemplaryembodiments, the slots or voids 132 extend through, and 80%-90% of theway along the axial length of, the conductive ion-collection surface130. It should be appreciated that, due to the slots or voids 132, theconductive ion-collection surface 130 forms a Faraday shield. Theconductive ion-collection surface thus reduces, and ideally eliminates,the time-varying electric field, generated by placing theradio-frequency signal on the radio-frequency antenna 140, frompenetrating into the plasma chamber portion 131 of the interior space106 that is enclosed or surrounded by the conductive ion-collectionsurface 130. At the same time, the slots or voids 132 in the conductiveion-collection surface 130 allow the time-varying magnetic fields topenetrate into the plasma chamber portion 131 of the interior space 106.

However, it should be appreciated that, in some exemplary embodiments,it may be desirable to allow some capacitive coupling to occur betweenthe antenna 140 and the plasma 108. Such capacitive coupling can be usedto ignite the plasma. It should further be appreciated that any otherknown or later developed ignition device or structure that is usable toignite the plasma 108 can be used. In such exemplary embodiments,capacitive coupling between the antenna 140 and the plasma 108 can besubstantially eliminated, and, potentially, completely eliminated.

As shown in FIG. 1, the conductive ion-collection surface 130 isconnected by a connection 162 to a relatively negative voltage source160. It should be appreciated that, in this context, the voltage on theconductive ion collection surface 130 need only be relatively negative,i.e., less than, compared to the voltage applied to the extraction ring150. Here, the difference in potential between the plasma and the ioncollection cylinder should be much greater than the electrontemperature, in eV, divided by the electron charge. Consequently, theconductive ion-collection surface 130 is at a negative potential withrespect to the extraction ring 150 and the plasma 108, and thus acts toattract the positively-charged ions that are present within the plasma108.

As shown in FIG. 1, the conductive ion-collection surface 130 has asurface area, or ion loss area, A_(i). It should be appreciated that theion loss area A_(i) depends upon the axial length of the conductiveion-collection surface 130, its shape and surface conformation, and thearea consumed by the slots or voids 132. In particular, the ion lossobtained by the conductive ion collection surface 130 is a function ofan effective ion loss area A_(i) that differs from the geometric A_(g)and is determined by the magnetic field B. It should be appreciatedthat, in various exemplary embodiments, it is generally desirable tomaximize the surface or ion loss area A_(i) of the conductiveion-collection surface 130. Maximizing the ion loss area A_(i) isdesirable as it provides the maximum electron extraction that is allowedby the electron loss area A_(e). It is generally desirable to increasethe ion collection area A_(i) because that determines the maximumelectron current that can be lost to the electron sheath. This occurswhen the relationship A_(e)/A_(i)≧√{square root over (m_(e)/m_(i))} issatisfied, i.e., total non-ambipolar flow is obtained. In variousexemplary embodiments, the cross-sectional shape of the conductiveion-collection surface 130 closely follows the cross-section of thenon-conductive exterior chamber 110. In various exemplary embodiments,the non-conductive exterior chamber 110 and the conductiveion-collection surface 130 are concentric cylinders.

The central aperture, hole or passageway 152 in the electron extractionring 150 allows the feed gas 102 and the electrons obtained from theplasma 108 to be emitted from the electron beam generating device 100 asthe electron beam 104. As shown in FIG. 1, the electron extraction ring150 is connected by a connection 166 to a local ground potential 164.

The conductive ion-collection surface 130 acts as a radial boundary forthe plasma 108 and acts as the location for the formation of an ionsheath, shown in FIGS. 5 and 6, that prevents electrons from leaking tothe walls of the conductive ion-collection surface 130 and/or thenon-conductive exterior chamber 110. In various exemplary embodiments,the cylindrical conductive ion-collection surface 130 has between 1 and8 or more axial slots or voids 132. It should be appreciated that, whilethere could be a larger number of slots, this would tend to decrease theion loss area A_(i) and increase the penetration of the time-varyingelectric fields. The axial slots or voids 132 allow the time-varyingmagnetic fields, generated by placing a radio-frequency signal onto theradio-frequency antenna 140, into the plasma chamber portion 131 of theinterior space 106 that is enclosed by the cylindrical conductiveion-collection surface 130. At the same time, the conductiveion-collection surface 130 limits the time-varying electric fieldsgenerated by placing the radio-frequency signal onto the radio-frequencyantenna 140 from entering into the plasma chamber.

The electron extraction ring 150 creates an axial boundary condition,limiting the ability of the ions and the feed gas 102 to exit theinterior space 106 through the central aperture, hole or passageway 152.In various exemplary embodiments, the electron extraction ring 150creates a potential reference for the plasma 108 somewhere near thepotential of the plasma 108. An electron loss area A_(e) is establishedwithin the aperture 152 of the electron extraction ring 150. Theelectron loss area A_(e) can be as large as the area of the aperture152. However, an electron sheath usually forms near the extraction ring150. As indicated above, the electron extraction ring 150 can extendthrough the extraction end wall 118. In various exemplary embodiments,the electron extraction ring 150 extends into the interior space 106from the extraction end wall 118 within the conductive ion-collectionsurface 130.

The plasma 108 is formed by supplying the feed gas 102 from a mass flowcontroller (not shown) to at least the interior space 106 through thegas supply tube 116. In various exemplary embodiments, the feed gas 102is argon (Ar), xenon (Xe), or other noble gas. However, it should beappreciated that, in various other exemplary embodiments, the feed gas102 can be any desired elemental gas, gas mixture or the like. Invarious exemplary embodiments, the feed gas 102 flows from the gassupply tube 116 into a source, or plasma, region of the interior space106 where the feed gas 102 is excited by the radio-frequency antenna 140to form the plasma 108. In the exemplary embodiment shown in FIG. 1,this source region is the plasma chamber portion 131. It should beappreciated that the feed gas 102 can be supplied into the interiorspace 106 using the gas supply tube 116 or any other known orlater-developed device, structure or system that is capable of supplyingthe feed gas 102 into the interior space 106.

FIGS. 2-4 show plan end views and a perspective view, respectively ofthe exemplary embodiment of the electron beam generating device 100shown in FIG. 1. In particular, FIGS. 2-4 illustrate that, in variousexemplary embodiments, the permanent magnet or electromagnet 122 has acylindrical space that allows the cylindrical non-conductive exteriorchamber 110 to extend through the first steady-state magnet 122. Thisfirst steady-state magnet 122 produces an expanding magnetic field inthe region of the radio-frequency antenna 140 and the electronextraction ring 150. The expanding magnetic field creates a cusp in themagnetic field at the point where the north end of the firststeady-state magnet 122 is axially adjacent to the end of thecylindrical conductive ion-collection surface 130. It should beappreciated that the cusp in the magnetic field is not critical.However, the effective ion contact area, A_(i), created by the magneticfield needs to be large enough to provide the desired electron current,following Eqs. (3)-(6) set forth below. The magnetic field should berelatively uniform near the extraction ring.

This solenoidal magnetic field ensures that the electrons follow themagnetic field lines that pass through the central aperture, hole orpassageway 152, i.e., the exit region, of the electron extraction ring150. It should be appreciated that, for spacecraft/satellites and otherspace and/or weight limited structures, permanent magnets are relativelymore useful than electromagnets for the electron beam generating device100, as they do not require a power source for continued operation (incontrast to electromagnets) and are relatively light weight compared tothe DC power source that would be required by electromagnets. It shouldbe appreciated that electromagnets provide an option where a magneticcusp does not exist. Electromagnets can better adjust the strength ofthe magnetic field, which may increase the amount of extractableelectron current.

It should be appreciated that, in general, electron or ion sheaths arenon-neutral regions that usually form at plasma boundaries to balancelosses of electrons and ions born by ionization within the plasma. Anelectron sheath is a non-neutral region at the boundary of a plasma thatonly contains electrons for potential steps much greater than T_(e)/e(the plasma temperature/electron charge ratio), formed in order toconserve particle flux for the plasma as a whole. An electron sheathexhibits a positive potential step with respect to the bulk plasmapotential. Normally, electron sheaths can exist near positively-biasedLangmuir probes, which extract small electron currents from the plasma.However, electron beam generating devices according to this inventioncan use an electron sheath to extract a significant electron currentfrom the plasma. Electron sheaths are normally only present 1) nearsmall probes when such small probes are biased more positively than theplasma potential or 2) at electron emitting surfaces inweakly-collisional, low-pressure plasmas. The inventors have determined,experimentally, that an electron sheath can collect all electronsproduced by ionization if sufficient ion loss area A_(i) is provided forthe ions, according to Eq. (4), below.

If all of the boundaries are identical, then ambipolar flow of theelectrons and ions from the plasma is obtained. Ambipolar flow refers toboth the ions and electrons flowing and reaching a physical boundarytogether. In such ambipolar flows, the ion loss and the electron lossare balanced at each point on the boundary. In contrast, innon-ambipolar flow, the particles flow from the plasma to theplasma-sheath boundary together, but they do not leave the other end ofthe sheath, i.e., traverse the sheath, with the same current.

In contrast, if several different (i.e., non-identical) boundaries arepresent, then at least some non-ambipolar flow is created. Suchnon-ambipolar flow implies that, at least some points along theboundary, the electron and ion flows do not balance. That is, in suchnon-ambipolar flow, at some points along the boundary, the electron fluxis greater than the ion flux. An electron sheath may exist at suchpoints. In contrast, at various other boundary points, the ion fluxexceeds the electron flux. An ion sheath exists at such points. Itshould be appreciated that an ion sheath will also exist for normalambipolar flow.

With non-ambipolar flow, while the electron and ion fluxes do notbalance locally, they continue to balance overall. If there are nopoints within the plasma boundary where both electrons and ions flow atthe same time, the flow within the plasma can be referred to as totalnon-ambipolar flow. By insuring total non-ambipolar flow, all of theelectrons in the plasma remain available for extraction from the ionbeam generating device.

FIG. 5 shows in greater detail a first exemplary embodiment of theelectron extraction end of the electron beam generating device 100 shownin FIGS. 1-4. FIG. 6 shows in greater detail the supply end of theelectron beam generating device 100 shown in FIGS. 1-4. As shown in FIG.5, an electron sheath 136 is formed within the conductive ion-collectionsurface 130 that is adjacent to the grounded electron extraction ring150. It should be appreciated that the plasma potential should bebetween the extraction ring potential and the ion collection surfacepotential for an electron sheath to exist. In contrast, an ion sheath134 is formed adjacent to the interior surface of the conductiveion-collection surface 130. Thus, for a cylindrical conductiveion-collection surface 130, the ion sheath 134 will be an annulusclosely following the interior surface of the conductive ion-collectionsurface 130.

FIG. 7 shows in greater detail a second exemplary embodiment of theelectron extraction end of the electron beam generating device 100 shownin FIGS. 1-4. As shown in FIG. 7, in this second exemplary embodiment ofthe electron extraction end, the insulating end cap 118 of thenon-conductive exterior chamber 100 is replaced with a conductive endcap 154 having an exit aperture 155. Additionally, the conductiveion-collection surface 130 is provided with a conductive end cap 138.Furthermore, an insulating member or plate 156, having an aperture 157,is provided between the end caps 138 and 154. In the exemplaryembodiment shown in FIG. 7, the insulating member or plate 156 and theend caps 138 and 154 are positioned such that the end caps 138 and 154are immediately adjacent to, or even touching, the insulating member orplate 156. This conductive end cap 138 contacts the conductiveion-collection surface 130, so that both the conductive end cap 138 andthe conductive ion-collection surface 130 are at the same potential. Invarious exemplary embodiments, the conductive end cap 138 can be aseparate element. In various other exemplary embodiments, the conductiveend cap 138 is an integral portion of the conductive ion-collectionsurface 130.

As shown in FIG. 7, the electron extraction aperture 152, which wasprovided in the electron extraction ring 150 in the first exemplaryembodiment of the extraction end of the electron beam generating device100, is now provided in the conductive end cap 138. Furthermore, theconductive end cap 154 is, in effect, the electron extraction ring.However, it should be appreciated that, in this exemplary embodiment,the electron extraction aperture 152 is located in the conductive endcap 138, as the electron sheath forms in and/or adjacent to the aperturein the conductive end cap 138, rather than the aperture 156 in theconducive end cap 154. Additionally, as shown in FIG. 7, in variousexemplary embodiments, the apertures 152, 155 and 157 are generallyequal in size and location.

It should be appreciated that, in various other exemplary embodiments,one or more appropriately-sized gaps can be provided between the outersurface of the conductive end cap 138 and the inner surface of theinsulating member or plate 156 and/or the inner surface of theconductive end cap 154 and the outer surface of the insulating member orplate 157. In still other exemplary embodiments, the insulating memberor plate 156 can be removed completely, with an appropriately-sized gapprovided between the end caps 138 and 154. This gap allows the twoconductive end caps 138 and 154 to be at different potentials.

It should be appreciated that each of the exemplary embodimentsdiscussed above with respect to FIG. 7 allows the extracted electronbeam 104 to be accelerated away from the electron beam generating device100. Thus, it should be appreciated that, in this second exemplaryembodiment, the conductive end cap 154 replaces the extraction ring 150.However, the acceleration of electrons and the extraction of electronsis still provided by the electron sheath that is located in and/oradjacent to the electron extraction aperture 152. Furthermore, theelectron extraction aperture 152 and the apertures 155 and 157 in theconductive end cap 154 and the insulating member or plate 156,respectively, are aligned to allow the extracted electron beam 104 andthe neutral gas 102 to exit both the plasma chamber 131 and the electronbeam generating device 100.

To maintain steady-state operation, the amount of electron loss from thesource plasma 108 must be balanced by an equal amount of ion loss fromthe source plasma 108. Because electrons and ions are born at an equalrate within the plasma 108 created by the time-varying radio-frequencysignal applied to the radio-frequency antenna 140, it is desirable toprovide an efficient loss mechanism for the positively-charged ions, sothat an equal amount of electron current can be extracted from theplasma 108. It should be appreciated that ion and electron losses, gasutilization rates, plasma density and plasma potential effects allaffect the total amount of electron current that can be extracted fromthe electron beam generating device 100. It should be appreciated that,in general, the electron sheath 136 can extract almost all of the randomelectron current from the plasma 108 that is incident upon the electronsheath 136. In particular, the random electron flux J_(0e), directedtowards the electron sheath 136 in a weakly magnetized plasma, at theedge of the electron sheath 136 is:

$\begin{matrix}{J_{0e} = {\frac{n_{0e}e\;\alpha_{e}}{4}\sqrt{\frac{8T_{e}}{\pi\; m_{e}},}}} & (1)\end{matrix}$where:

n_(0e) is the electron density in the plasma;

e is the electron charge; equal to 1.60217646×10⁻¹⁹ Coulombs;

α_(e) is an electron factor that takes into account the drop in electrondensity associated with potential dips preceding the electron sheath;

T_(e) is the temperature of the plasma electrons, measured in electronvolts (eV); and

m_(e) is the electron mass.

At the same time, the ion flux J_(0i) at the ion sheath edge is:

$\begin{matrix}{{J_{0i} = {n_{0i}e\;\alpha_{i}\sqrt{\frac{T_{e}}{m_{i}}}}},} & (2)\end{matrix}$where:

n_(0i) is the ion density and should be equivalent to the electrondensity n_(0e) for singly-ionized ions;

α_(i) is an ion factor that takes into account the drop in ion densityin the presheath near the conductive ion-collection surface.

T_(e) is the temperature of the plasma electrons measured in electronvolts; and

m_(i) is the ion mass.

For total non-ambipolar flow, the ratio of the electron loss area to theion loss area is found by setting I_(e)=I_(i), where I_(c)=J_(0e)A_(e)and I_(i)=J_(0i)A_(i), and the electron flux J_(0e) to the ion fluxJ_(0i) associated with electrons created by ionization can be obtainedby combining Eq. (1) and Eq. (2) and is approximately equal to:

$\begin{matrix}{\frac{A_{e}}{A_{i}} \approx {\frac{\alpha_{i}}{\alpha_{e}}{\sqrt{\frac{2\;\pi\; m_{e}}{m_{i}}} \cdot}} \approx \sqrt{\frac{m_{e}}{m_{i}}}} & (3)\end{matrix}$

A limit to the existence of an electron sheath is provided by thecondition that the ion loss area A_(i) be balanced by the electron lossarea A_(e) it should be appreciated that, when the ion loss area is toosmall, the electron beam device 100 will still work, but this reducedion loss area A_(i) reduces the amount of electron current that can beproduced by forming a plasma potential dip preceding the electronsheath, as discussed below. Assuming all of the electrons are lost atthe electron sheath 136, then:

$\begin{matrix}{\frac{A_{e}}{A_{i}} \approx {\sqrt{\frac{m_{e}}{m_{i}}}.}} & (4)\end{matrix}$assuming the electrons are radially confined. It should be appreciatedthat an electron sheath will form without a potential dip if:

$\begin{matrix}{A_{e} \leq {A_{i}{\sqrt{\frac{m_{e}}{m_{i}}}.}}} & (5)\end{matrix}$However, an electron sheath will form with a potential dip in front ofit if:

$\begin{matrix}{A_{e} > {A_{i}{\sqrt{\frac{m_{e}}{m_{i}}}.}}} & (6)\end{matrix}$

It should be appreciated that, for large electron loss areas A_(e), theelectron sheath 136 is no longer a viable solution. For suchsufficiently large electron loss areas A_(e), only a plasma potentialmore positive than the grounded electrode potential, combined with anion sheath 134, can provide the necessary balance of electron and ionlosses.

It should be appreciated that the net electron loss in traditionaldevices, such as hollow cathodes, equals the sum of the electrons bornby ionization within the plasma, and electrons injected into the plasmaby thermionic emission at cathodes, secondary electron emission and thelike. However, electron loss within the electron beam generating device100 only comes from electrons that are born by ionization and perhapssecondary emission. It should also be appreciated that if the electronloss area, A_(e), is too large, as defined in Eq. (6), then the electronsheath will have a potential dip that reduces the extracted electroncurrent to balance that of the extracted ion current in the device 100.As outlined above, such potential dips occur when the electronextraction area A_(e) is too large, i.e., the relationship defined inEq. (6).

In one exemplary electron beam generating device built according to theabove-outlined discussion of various exemplary embodiments of electronbeam generating devices according to this invention, a cylindrical pyrexchamber has a diameter of 7.5 centimeters and a length of 60 centimetersand was placed within ferrite permanent magnets. A hollow graphitecylinder 7.5 centimeters in diameter and 19 centimeters long was placedwithin the pyrex chamber and biased at a value between −5V to about−200V compared to the potential on the extraction ring. An electricallygrounded 1.25 centimeter diameter graphite ring was placed inside aninsulating boron nitride disc and mounted in one end of the cylindricalpyrex chamber. The hollow graphite cylinder was placed adjacent to theelectron extraction ring. A single-turn, 0.25-inch-diameter,water-cooled copper pipe was placed, as the radio-frequency antenna,around the pyrex chamber toward the extraction end of the pyrex chamber.In this exemplary embodiment, the grounded electron extraction ringexperienced a magnetic field of 72 Gauss.

In this exemplary operating electron beam generating device, ions arelost to the 7.5 centimeter diameter graphite cylinder, which has an ionloss area of 425 cm². In contrast, the electron loss area A_(e) isrestricted to the central aperture, hole or passageway in the 1.25centimeter-diameter graphite extraction ring, which has an area of 1.23cm². The 1.23 cm² electron loss area A_(e) implies that an ion loss areaof at least about 350 cm² for argon (Ar) and at least about 640 cm² forxenon (Xe) (assuming the electron and ion factors have values ofα_(e)˜1, and α_(i)˜0.5) would be needed. When the plasma in thisexemplary operating electron beam generating device is operated with anargon feed gas and a plasma density of 5×10¹²/cm³, a 15 A electroncurrent can be extracted through the central aperture, hole orpassageway having an electron loss area of 1.23 cm². The 15 A currentwas extracted with the following parameters: 1000 W RF power at afrequency of 13.56 MHz, −50V bias on the ion collection cylinder, 0Vbias (grounded) on the extraction ring, an electron loss area of 1.23cm², and an Ar neutral gas flow rate of 15 sccm with an aluminumion-collection cylinder. 10 A electron extraction current was obtainedwith a graphite ion-collection cylinder with somewhat differentdimensions.

A positively-charged ion born within an electron beam generating deviceaccording to this invention is transported from the bulk plasma througha presheath and then to the ion sheath, where it contacts the conductiveion-collection surface and picks up an electron, converting thepositively-charged ion into a neutral atom. The neutral atom is thenfree to travel back into the bulk plasma within the plasma chamberportion to be re-ionized. At any one time, only a small fraction, on theorder of about 1 atom out of 1000, of the neutral gas is ionized.However, as described above, each neutral atom may be recycled manytimes, such as, for example, up to 20 times or more, before that neutralatom finally exits the electron beam generating device according to thisinvention.

Typically, the neutral atoms will exit through the aperture in theelectron extraction ring. However, in contrast to the neutral atoms, thepositively-charged ions see a potential barrier at the aperture in theelectron extraction ring of the electron beam generating device so thatonly neutrals and electrons can leave the interior chamber. Reusing theneutral gas atoms in this way is possible because the positively-chargedions, in contrast to ion thrusters, are not being extracted through theexit aperture. That is, when extracting ions, as in an ion thruster, theion outflow rate can never exceed the neutral inflow rate. However,because the electron beam generating device according to this inventionis an electron source that extracts electrons, the electron outflow ratecan be many times the neutral gas inflow rate. In general, the ratio ofextracted electrons to the amount of neutral gas exiting the electronbeam generating device depends on the plasma density, the electrontemperature, the flow rate of neutral gas into the interior chamber, andthe size of the electron extraction aperture.

It should be appreciated that, if higher plasma densities are obtained,a higher electron current can be extracted from the electron beamgenerating device or a correspondingly smaller electron loss area A_(e)can be used. Of course, it should be appreciated that, by using asmaller electron loss area A_(e), a correspondingly smaller ion lossarea A_(i) for the conductive ion-collection surface can be used. Itshould be appreciated that the electron extraction current cannot exceedthe ion extraction current that is controlled by the ion loss areaA_(i). It should be appreciated that using a smaller electron loss areaA_(e) has the advantage of lower neutral gas losses.

For electron beam generating devices that are used as chargeneutralizers in satellite and/or spacecraft applications, it isbeneficial to produce the plasma 108 using a method that creates thelargest fraction of ionization possible, so that the neutral feed gas102 is not wasted. For example, if the plasma source were 100% efficientin ionizing a neutral gas 102, as it flows through the interior space106, and each neutral atom is used only once before it touches the ioncollection cylinder, a feed gas flow rate of 1 sccm (standard cubiccentimeter per minute) of argon allows obtaining (is equivalent to)0.072 A of extraction current.

However, the inventors have experimentally determined that, when thefeed gas 102 is neutral argon, the neutral argon feed gas 102 is moreefficiently utilized to create extraction current at flow rates betweenabout 2.5 sccm and about 15 sccm. At these flow rates, the amount ofextraction current that can be obtained corresponds to using every atomapproximately 14 times as it passes from the gas supply tube 116,through the plasma 108 and out through the central aperture, hole orpassageway 152 into a target region. In conventional plasma-basedelectron sources, plasma ions and electrons are both extracted. Asindicated above, in various electron beam generating devices accordingto this invention, any ions that encounter the electron sheath arereflected. Furthermore, all ions encounter the ion-collection walls andare re-circulated as neutrals. As set forth in Eqs. (1) and (2), theamount of extractable current is linear with the plasma density, which,in turn, increases with radio-frequency power applied to theradio-frequency antenna 140.

FIG. 17 is a graph showing the results of an experiment using theabove-outlined exemplary embodiment of an electron beam generatingdevice according to this invention. As shown in FIG. 17, when theradio-frequency power is at or above about 400 W, for most neutral gassupply flow rates into the electron beam generating device, at leastabout 1 electron is extracted for each neutral atom lost from theelectron beam generating device through the aperture in the electronextraction ring. Additionally, as the radio-frequency power increases,for any flow rate, the electron current-particle current ratioincreases. As indicated above, at an RF power of 1000 W and flow ratesbetween about 3 sccm and about 15 sccm, ratios of about 10 to about 20were obtained.

FIG. 8 shows a second exemplary embodiment of an electron beamgenerating device 200 according to this invention. As shown in FIG. 8,in this second exemplary embodiment, the electron beam generating device200 includes a non-conductive exterior chamber 210, at least onesteady-state magnet 220, a conductive ion-collection surface 230, aradio-frequency antenna 240, an electron extraction ring 250, and anegative voltage source 260. However, in contrast to the first exemplaryembodiment of an electron beam generating device 100 shown in FIGS. 1-7,in the second exemplary embodiment of the electron beam generatingdevice 200 shown in FIG. 8, the at least one steady-state magnet 220 isplaced at the rear of the device, rather than around the non-conductiveexterior chamber 210.

As shown in FIG. 8, the non-conductive exterior chamber 210 includes anon-conductive chamber surface 212, supply and extraction end walls 214and 218, respectively and the gas supply tube 216. As in the firstexemplary embodiment, the gas supply tube 216 passes through theinterior of the at least one steady-state magnet 220 and extends throughthe supply end wall 214, while the electron extraction ring 250 isattached to the extraction end wall 218. The gas supply tube 216supplies a feed gas 202 to an interior space 206 within thenon-conductive exterior chamber 210. The feed gas 202 is converted intoa plasma 208 within the conductive ion-collection surface 230. Electronsextracted from the plasma 208 are ejected from the electron beamgenerating device 200 through the central aperture, hole or passageway252 in the electron extraction ring 250.

A radio-frequency signal is applied to the radio-frequency antenna 240.The electromagnetic field generated in response to placing this RFsignal on the radio-frequency antenna 240 is inductively coupled toinductive or helicon modes through the slots or voids 232 formed in theconductive ion-collection surface 230 to the plasma 208 within theconductive ion-collection surface 230. The negative voltage source 260is connected by a conductor 262 to the conductive ion-collection surface230. The electron extraction ring 250 is connected by a conductor 266 toa local reference ground potential 264.

FIG. 9 shows a plan exterior view of the supply end wall 214, the atleast one steady-state magnet 220 and the gas supply tube 216 of theelectron beam generating device 200. As shown in FIG. 9, the at leastone steady-state magnet 220 is a single, annularly-shaped permanentmagnet. However, it should be appreciated that an electromagnet could beused as the steady-state magnet 220. Additionally, two or more separatemagnet segments could be used to implement the annular steady-statemagnet 220.

As shown in FIGS. 8 and 9, in various exemplary embodiments, the annularsteady-state magnet 220 is relatively thin, with a central void that issubstantially larger than the gas supply tube 216. In such exemplaryembodiments, the gas supply tube 216 can be placed along the axis of themagnet 220 and the non-conductive exterior chamber 210. However, the gassupply tube 216 could be placed anywhere within the interior of themagnet 220. In various other exemplary embodiments, the annularsteady-state magnet 220 is thick, with a central passageway that is onlyslightly larger than the gas supply tube 216. In such exemplaryembodiments, the gas supply tube 216 is typically placed along the axisof the magnet 220 and the non-conductive exterior chamber 210.

FIG. 10 shows a variation of the second exemplary embodiment of theelectron beam generating device 200 according to this invention, wherethe steady-state magnet 220 is a solid cylinder or other solid shape andthe gas supply tube 216 extends through the sidewall 212 of thenon-conductive exterior chamber 210 rather than the end wall 214. Inthis exemplary embodiment, the gas supply tube 216 also extends throughthe side wall, rather than the end wall, of the conductiveion-collection surface 230. It should be appreciated that the gas supplytube 216 can be located anywhere along the axial length of thenon-conductive exterior chamber 210.

One advantage provided by the magnetic field generated by the one ormore steady state magnets 120 or 220 is that it increases the plasmadensity. The magnetic field also reduces the relative electron losses tothe extraction ring, while allowing the electron sheath to form at ornear the extraction aperture. This makes the electron beam extractiondevice more efficient and increases the maximum current that can beproduced by the electron beam extraction device.

FIG. 18 is a graph showing the results of an experiment using theexemplary embodiment of an electron beam generating device 100 accordingto this invention. The graph shown in FIG. 18 demonstrates theimportance of the presence of the magnetic field at the exit aperture ofthe electron beam extraction device. As shown in FIG. 18, as themagnetic field strength increases, the extraction current I_(e)increases. This occurs because the plasma density increases as themagnetic field strength increases. At the same time, as shown in FIG.18, the current I_(ring) that is lost to the electron extraction ringstays substantially constant even as the magnetic field strengthincreases. As a consequence, the fraction of the extraction current thatis lost to the electron extraction ring I_(ring) decreases significantlywith increasing magnetic field strength.

FIG. 11 is a schematic view of one exemplary embodiment of an electronbeam generating device 300 and associated antenna drive circuitryaccording to this invention. As shown in FIG. 11, a negative voltagesource 360 is connected by a conductor 362 to a conductiveion-collection surface 330 that is contained within the electron beamgenerating device 300. The negative voltage source 360 is also connectedby a conductor 363 to a reference ground potential 364. An electronextraction ring 350 is connected by a conductor 366 to the groundpotential 364 as well. A radio-frequency antenna 340 is placed adjacentto, and around, the conductive ion-collection surface 330. Ends of theradio-frequency antenna 340 are connected by signal lines 396 to amatching circuit 390. A function generator 370 generates and outputs atime-varying radio-frequency electric signal on a signal line 372 to aradio-frequency amplifier 380. The radio-frequency amplifier 380amplifies the radio-frequency time-varying electric signal output by thefunction generator 370. The radio-frequency amplifier 380 outputs theamplified radio-frequency signal on a signal line 382 to the matchingcircuit 390.

FIGS. 12-15 illustrate a variety of additional radio-frequency antennadesigns that can be used in place of the radio-frequency antennas 140,240 and 340 shown in FIGS. 1, 8 and 11. It should be appreciated thatdifferent ones of these radio-frequency antennas are appropriate fordifferent plasma densities and/or different operational modes, such asthe inductive coupled mode and the helicon mode.

FIG. 16 illustrates one exemplary embodiment of a method for generatingan electron beam and using the generated electron beam to neutralize apositively-charged ion stream according to this invention. It should beappreciated that this is only one exemplary use of an electron beamgenerating device according to this invention, which could be used forany other appropriate known or later-developed use, as outlined above.In particular, FIG. 16 illustrates the actions that occur to a givenquantity of gas. It should be appreciated that, as gas is continuallyintroduced into the device, all of steps S100-S700 occur simultaneouslyrelative to different quantities of gas or electrons. As shown in FIG.16, beginning in step S100, operation of the method continues to stepS200, where gas is introduced into a vacuum chamber at very lowpressures and at a defined flow rate. Then, in step S300, the introducedgas is inductively ionized to form a plasma. Then, in step S400, an ionpotential V_(I) is applied to an ion collection cylinder, while areference ground potential is applied to an electron extraction ring ofthe electron beam generating device. Operation then continues to stepS500.

In step S500, a non-ambipolar flow of ions towards the ion collectioncylinder and electrons towards the electron extraction ring is created.In various exemplary embodiments, this non-ambipolar flow is a totalnon-ambipolar flow. Next, in step S600, electrons are ejected throughthe electron extraction ring while neutral gas passes through theelectron extraction ring. Then, in step S700, the extracted electronsare combined with a positively-charged ion stream to neutralize thepositively-charged ions in the ion stream. Operation then continues tostep S800.

In step S800, a determination is made whether to continue introducingthe supply gas into the vacuum chamber. If so, operation jumps back tostep S500 and steps 500-700 are repeated. In contrast, if additional gasis not to be introduced into the vacuum chamber, operation continues tostep S900, where operation of the method ends.

In various exemplary embodiments, the amount of electron current can beextracted from an electron beam extraction device according to thisinvention varies linearly with the plasma density. In turn, the plasmadensity increases as the radio-frequency power increases. In experimentsperformed by the inventors, the extracted current increase linearly withincreases in the radio-frequency power and did not indicate a saturationpoint at high radio-frequency powers, indicating room for futureprogress. At radio-frequency powers between 60 W and 90 W, the plasmadid not visually fill the entire conductive ion-collection surface, thusdecreasing the effective ion loss area A_(i), resulting in decreased ioncollection current.

As the DC bias on the conductive ion-collection surface was decreasedfrom 0V to −60V, the conductive ion-collection surface repelled a largernumber of electrons away from the conductive ion-collection surface.This increased the local plasma density, which then allowed theconductive ion-collection surface to collect more ion current. Themeasured electron current from an electron beam extraction deviceaccording to this invention agreed closely with the total amount of ionextraction current. This shows that, in some exemplary embodiments, allof the electrons that are lost within the electron beam extractiondevice according to this invention are lost through the electronextraction ring.

One complication to understanding the electron extraction from theplasma source is the plasma potential difference between the plasma sideand the extraction side. Regardless of the bias on the conductiveion-collection surface within the electron beam extraction deviceaccording to this invention, the plasma potential of the target sideremained more positive than the potential of the plasma source region.At the same time, the plasma potential within the plasma source regionremained more positive than the potential on the conductiveion-collection surface.

In experiments, the respective plasma source and conductiveion-collection surface potentials were −10V and −50V. It should beappreciated that, in these experiments, the extraction ring wasgrounded. Accordingly, this allowed ion loss through an ion sheath tothe conductive ion-collection surface within the source region.Similarly, the plasma potential in the region around the extractionaperture remained more positive than the plasma potential in the region.This indicated the existence of an electron sheath at the boundarybetween the plasma region and the electron extraction ring and aperturethat is extracting electrons from the plasma.

As discussed above, the conductive ion-collection surface acts as aFaraday shield. By using the conductive ion-collection surface as aFaraday shield, the plasma potential did not fluctuate significantly. Incontrast, when the ion-collection surface/Faraday shield was modified inthe exemplary embodiment shown in FIG. 1, so that it was no longerunderneath the radio-frequency antenna, a large AC fluctuating plasmapotential was created. Without the conductive ion-collectionsurface/Faraday shield, there was significant capacitive couplingbetween the radio-frequency antenna and the plasma. As a result, theplasma potential oscillated back and forth, with a peak-to-peakoscillation of over 100V. Canceling the fluctuating plasma potential byusing the conductive ion-collection surface/Faraday shield, is, invarious exemplary embodiments, beneficial, as it allows for larger andmore stable extraction currents to be obtained.

The conductive ion-collection surface provided the necessary ion lossarea A_(i), while the smaller grounded electron extraction ring was usedto extract the electrons through an electron sheath into a targetregion. It is possible, using an electron beam extraction deviceaccording to this invention, to scale the extracted electron currentbased on the total amount of the ion loss area A_(i) of the conductiveion-collection surface that is located within the source plasma. Thetotal amount of extracted electron current from an electron beamextraction device according to this invention is ultimately limited byone or more of the ion loss area A_(i), the electron loss area A_(e),the neutral gas flow rate, and the radio-frequency power.

While this invention has been described in conjunction with theexemplary embodiments outlined above, various alternatives,modifications, variations, improvements and/or substantial equivalents,whether known or that are or may be presently foreseen, may becomeapparent to those having at least ordinary skill in the art.Accordingly, the exemplary embodiments of the invention, as set forthabove, are intended to be illustrative, not limiting. Various changesmay be made without departing from the spirit or scope of the invention.Therefore, the invention is intended to embrace all known or earlierdeveloped alternatives, modifications, variations, improvements and/orsubstantial equivalents.

1. An electron beam generating device, comprising: a first chamberusable to contain a feed gas and a plasma formed using the feed gas, thefirst chamber having a first end wall having an opening through whichthe feed gas is able to exit the first chamber; a gas supply tube usableto supply the feed gas into the first chamber, the gas supply tubeextending at least into the first chamber; a conductive surface providedwithin the first chamber, the conductive surface electrically connectedto an at least relatively negative potential source that is usable toplace an at least relatively negative potential onto the conductivesurface; at least one radio-frequency antenna placed around the firstchamber and usable to ionize the feed gas within the first chamber toform a plasma; and an electron extraction ring provided adjacent to thefirst end wall of the first chamber and relative to the opening in thefirst end wall, the electron extraction ring electrically connected to areference ground potential source and having an electron extractionaperture, wherein: the potential placed on the conductive surface isnegative relative to the plasma and the electron extraction ring; atleast an electron sheath forms within the first chamber relative to theelectron extraction ring, the electron sheath permitting electrons andneutral particles to pass out of the plasma toward the electronextraction ring and preventing positively-charged ions from movingtowards the electron extraction ring; and the electron extraction ringextracts and outputs at least a beam of electrons from the electron beamgenerating device.
 2. The electron beam generating device of claim 1,further comprising at least one magnet located adjacent to the firstchamber, the at least one magnet arranged such that a generallysolenoidal magnetic field extends along an axial direction of the firstchamber.
 3. The electron beam generating device of claim 2, wherein theat least one magnet is at least one permanent magnet.
 4. The electronbeam generating device of claim 2, wherein the at least one magnet is atleast one electromagnet.
 5. The electron beam generating device of claim4, wherein the at least one electromagnet generates a substantiallytime-constant magnetic field.
 6. The electron beam generating device ofclaim 2, wherein the at least one magnet is a cylindrical-prism-shapedor polygonal-prism-shaped magnet having a central passage extendingalong the axial direction of the magnet, the first chamber extendingthrough the central passage such that the at least one magnet extendsaround the first chamber.
 7. The electron beam generating device ofclaim 2, wherein the at least one magnet is a cylindrical-prism-shapedor polygonal-prism-shaped magnet, the at least one magnet positionedadjacent to a second end wall of the first chamber.
 8. The electron beamgenerating device of claim 7, wherein the at least one magnet is acylindrical-prism-shaped or polygonal-prism-shaped magnet having atleast one passage extending along the axial direction of the magnet, thegas supply tube extends through at least one of the at least one passagein the magnet and at least the second end wall of the first chamber. 9.The electron beam generating device of claim 2, wherein the solenoidalmagnetic field generated by the at least one magnet increases a plasmadensity of the plasma formed within the first chamber.
 10. The electronbeam generating device of claim 2, wherein the solenoidal magnetic fieldgenerated by the at least one magnet improves a uniformity of theelectron beam output by the electron beam generating device.
 11. Theelectron beam generating device of claim 2, wherein the solenoidalmagnetic field generated by the at least one magnet decreases a fractionof electron current that is drawn to the electron extraction ring. 12.The electron beam generating device of claim 2, wherein: a time-varyingelectric signal is applied to the at least one radio-frequency antennato generate time-varying electric and magnetic fields around the atleast one radio-frequency antenna; and a strength of the solenoidalmagnetic field generated by the at least one magnet and a power of thetime-varying magnetic field generated by the at least oneradio-frequency antenna are sufficient to excite helicon waves withinthe plasma formed in the first chamber.
 13. The electron beam generatingdevice of claim 2, wherein the solenoidal magnetic field extends axiallythrough the electron extraction aperture and is substantially uniformacross the electron extraction aperture.
 14. The electron beamgenerating device of claim 1, wherein a plurality of electrons areoutput from the electron beam generating device for each feed gasparticle output from the electron beam generating device.
 15. Theelectron beam generating device of claim 1, wherein a uniform plasmapotential forms across the area of the electron extraction aperture. 16.An electron beam generating device, comprising: a first chamber usableto contain a feed gas and a plasma formed using the feed gas, the firstchamber having a first end wall having an opening through which the feedgas is able to exit the first chamber; a conductive chamber providedwithin the first chamber, the conductive chamber having at least acircumferential wall than extends circumferentially within the firstchamber, a first end wall and an electron extraction end wall, anaperture formed in the electron extraction end wall that allowselectrons and neutral particles to exit the conductive chamber, theconductive chamber electrically connected to an at least relativelynegative potential source that is usable to place an at least relativelynegative potential onto the conductive chamber; a gas supply tube usableto supply the feed gas into the conductive chamber, the gas supply tubeextending into the first chamber and the conductive chamber; at leastone radio-frequency antenna placed around the first chamber and usableto ionize the feed gas within the conductive chamber into the plasma;and a conductive end wall of the first chamber provided adjacent to theextraction end wall of the conductive chamber, the conductive end wallhaving a second aperture located relative to the aperture in theextraction end wall, wherein: at least an electron sheath forms withinthe first chamber relative to the aperture provided in the extractionend wall, the electron sheath permitting electrons and neutral particlesto pass out of the plasma toward the aperture provided in the extractionend wall and preventing positively-charged ions from moving towards theaperture provided in the extraction end wall; and the extraction endwall extracts and outputs at least a beam of electrons from the electronbeam generating device.
 17. A method for generating a beam of electrons,comprising: supplying an amount of feed gas to into a plasma region of afirst chamber, the first chamber enclosing the plasma region, aconductive surface provided within the first chamber around the plasmaregion and an electron extraction ring provided at one end of the firstchamber, a generally constant solenoidal magnetic field provided to thefirst chamber, a first potential applied to the electron extractionring, a relatively-negative second potential applied to the conductivesurface; applying a time-varying radio-frequency signal to at least oneantenna provided around the first chamber; generating time varyingradio-frequency electric and magnetic fields based on the time-varyingradio-frequency signal applied to the at least one antenna; ionizing atleast some of the supplied amount of feed gas to form a plasma withinthe plasma region, the plasma containing free electrons and freepositively-charged ions; attracting at least some of the free electronsto the electron extraction ring through an electron sheath; attractingat least some of the free positively-charged ions to the conductivesurface through an ion sheath; converting the attractedpositively-charged ions back into neutral feed gas particles, theneutral feed gas particles returning to the plasma region, where theyare available to be re-ionized into the plasma.